AMES, Iowa – Many people love tomatoes from the garden, fresh and delicious. But diseases that attack them and limit their potential are no fun to deal with.
Iowa State University Extension and Outreach horticulturists can help solve these issues and ensure a big garden tomato crop.
My tomato plants are wilting despite adequate rainfall. Why?
A vascular wilt may be responsible for the wilting of the tomato plants. The initial symptoms of Verticillium and Fusarium wilts are wilting of the plant foliage during the heat of the day. Affected plants often recover in the evening or overnight. Gradually, however, the wilting becomes progressively worse and many plants eventually die.
Verticillium and Fusarium wilt are caused by soil-borne fungi that invade tomato plants through injured roots. The fungi spread into the water-conducting tissue (xylem) in the stem and block the flow of water to the foliage. Foliage of affected plants turns yellow, then wilts and dies. A cut through the lower stem of a dead plant often reveals a brownish discoloration of the vascular tissue.
Nothing can be done for plants that have Verticillium or Fusarium wilt. Plants that die should be removed and destroyed. Crop rotation is of limited value as the vascular wilt fungi may survive in the soil for several years. The use of resistant cultivars is the most practical way for home gardeners to prevent losses due to vascular wilts. Resistant cultivars are available in seed catalogs and at garden centers. The letters V and F following the cultivar name denote cultivars that are resistant to Verticillium and Fusarium wilts.
Stalk borers are another possibility. The stalk borer is an insect that attacks a wide variety of plants including tomatoes, potatoes, peppers, corn, hollyhocks and dahlias. The larva (caterpillar) bores into the stem and tunnels inside the stalk. (The entrance hole is small and often difficult to locate.) Affected plants wilt and often die. However, stalk borer damaged plants may survive when given good care.
The stalk borer is a purple and cream striped caterpillar with a solid purple band around its body one-third of the way back from its head. It is an early season pest that moves from tall grassy weeds and occasionally attacks plants in the garden. An individual stalk borer may go from one plant to another, damaging several plants. The adult is an inconspicuous grayish brown moth.
Tomato plants that die should be pulled and destroyed. Destroying the plants may kill the stalk borers, as well. Cutting or mowing tall weedy areas around gardens also may help control the pest. Stalk borers cannot be effectively controlled with insecticides.
Black spots are developing on the bottom of many of the fruit on my tomato plants. What is the problem?
Blossom end rot is probably responsible for the blackish spots on the tomato fruit. Blossom end rot is a common problem on tomatoes. It appears as a brownish black spot on the blossom end (bottom) of the fruit. Secondary organisms invade the brownish black spot and cause the fruit to rot. Blossom end rot is most common on the earliest maturing fruit that ripen in July and early August.
Blossom end rot is caused by a calcium deficiency in the developing fruit. Wide fluctuations in soil moisture levels impair calcium uptake by the root system of the tomato plant. Excessive nitrogen fertilization also may contribute to blossom end rot.
To reduce blossom end rot, water tomato plants on a weekly basis during dry weather to provide a consistent supply of moisture to the plants. (Tomato plants require 1 to 1½ inches of water per week during the growing season.) Mulch the area around tomato plants to conserve soil moisture. Avoid over-fertilization. There is no need to apply calcium to the soil, as most Iowa soils contain more than adequate levels of calcium.
Pick and discard fruit affected with blossom end rot. Removing the affected fruit will allow the tomato plant to channel all of its resources into the growth and development of the remaining fruit.
Blossom end rot also can occur on pepper, eggplant, summer squash and watermelon.